The main objective of this proposed research is to understnd how cardiac adenylyl cyclase activity is regulated by hormones and other factors. New methods for measurement of adenylyl cyclase activity will be utilized to analyze important kinetic and physical parameters of this activity in particulate and solubilized preparations of rat heart adenylyl cyclase and fractions which effect the sensitivity of these enzymes to pinephrine and glucagon. Current effort includes an extensive study of the mechanisms of densensitization to catecholamines using baby hampster kidney fibroblast as a model system as well as isolated heart cells from embryonic and adult heart tissues. Beta adrenergic receptor binding using (H3) (negative) alprenolol will be correlated to adenylyl cyclase activity in these cells under a variety of conditions, cell treatments, and metabolic and synthetic inhibitors to test the mechanism of tachyphylaxis to these agents at the cellular level. We also plan to apply recently developed membrane isolation procedures (based on glass permeation and affinity chromatography) to the problem of separating cardiac membrane complexes containing hormone receptors (beta adrenergic receptors and glucagon receptors) linked to adenylyl cyclase activity. The results of this study should lead to an improved understanding of the regulation of cardiac function by hormones. BIBLIOGRAPHIC REFERENCES: Enhancement of Epinephrine Stimulated Adenylate Cyclase Activity by a Supernatant Fraction from Rat Heart (1975). Federation proceedings, 34: 260. R.B. Sanders and G.A. Robison.